A gas laser having an optically unstable laser resonator is disclosed, for example, by the slab laser of EP 0305893A2 and the coaxial laser of EP 0911922A1. The two gas lasers mentioned have laser resonators which are unstable in one direction. The slab laser constitutes a modification of a waveguide laser, where the gas volume is delimited in two directions by wall faces which constitute an optical waveguide. In the slab laser, the waveguide is provided only in one direction, whilst the laser beam is not delimited by wall faces in the second direction and can propagate freely. The coaxial laser has an unstable laser resonator in the azimuthal direction and a stable laser resonator with free beam propagation in the radial direction. In gas lasers, the laser resonator is typically closed with respect to the environment in a gas-tight manner, owing to the gaseous laser medium. In optically unstable laser resonators, the laser beam is discharged from the laser resonator via an aperture in or beside a resonator mirror. That decoupling aperture is typically closed by a zinc selenide (ZnSe) or diamond decoupling window so that the laser resonator is closed in a gas-tight manner.
Laser beams may be guided by optical fibers in solid-state and diode lasers. Gas lasers such as, for example, carbon dioxide (CO2) and Er:YAG gas lasers, typically cannot be guided by conventional glass fibers because, at wavelengths greater than 2 μm, the laser radiation is absorbed in an intensified manner and the losses increase drastically. Therefore, the beam guiding is carried out, in lasers having wavelengths greater than 2 μm, in free beam propagation using transmitting and reflective optical elements such as, for example, lenses and/or mirrors.